Silicone elastomer porous materials are used in a variety of fields; for example, in components of image-forming devices (e.g., copying machines and laser printers), including a developing roller, a toner-feeding roller, a transfer roller, and a cleaning roller. Also, silicone elastomer porous materials are used in paper-sheet-feeding rollers of copying machines, various types of printers, and plotters, as well as in fixing components (e.g., a fixing roller and a pressure roller).
Hitherto, porous materials have generally been produced by means of foaming action. In a technique for causing foaming action, a chemical foaming agent, a gas, or water is used as a foaming agent.
In most cases of production of a silicone elastomer porous material, such a foaming agent is also used. However, in conventional methods for producing such a silicone elastomer porous material, curing of silicone rubber and foaming are performed in parallel, and thus the resultant porous material has cells (pores) which are not uniform in size; i.e., their sizes considerably differ from one cell to another. In addition, difficulty is encountered in forming spherical cells having a small size.
In view of the foregoing, Patent Document 1 discloses a method for producing a silicone elastomer porous material by freezing a room-temperature-curable organopolysiloxane emulsion containing, for example, an organopolysiloxane having a silanol group, a specific cross-linking agent, a curing catalyst, and an emulsifier, and drying the frozen emulsion by sublimation of water without thawing. However, this method also encounters difficulty in producing a porous material having small cells of uniform size.
When a silicone elastomer porous material produced by use of a foaming agent is used in a fixing roller, since the porous material has large cells of non-uniform size, the fixing roller poses problems in that it exhibits inconsistent form upon heating, and, when torque is applied to the fixing roller, the roller is likely to break due to failure to achieve uniform distribution of the torque. When a porous material having large-sized cells is used in, for example, a pressure roller, the contours of the cells may appear on a formed image. Thus, demand has arisen for a silicone elastomer porous material having small cells of uniform size.
The present applicant previously applied for a patent on a closed-cell silicone elastomer porous material which is produced essentially from a water-in-oil emulsion containing water and a liquid silicone rubber material that forms a silicone elastomer through curing (see Patent Document 2).
However, on the basis of subsequent studies, the present inventors have found that the thus-produced silicone elastomer porous material has cells attributed to the water-in-oil emulsion, as well as cells attributed to bubbles entrained upon preparation of the water-in-oil emulsion, and thus the porous material is unsatisfactory in terms of properties (in particular, durability) required of a silicone elastomer porous material.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. H06-287348    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2005-206784